airfoil polar data extrapolation
Table of Contents
1 aifoil polar extrapolation
NACA0012 cl cd data 1 naca0018 polar data 2
- method 1
- Method 2
- extrapolate cl, cd up to the stall delay angle using the method proposed by Snel et al.5.
Further extrapolations beyond the stall delay angle were achieved using Viterna and Corrigan’s methodology for post stall predictions 6.
- empirical correction
- used by batten 2008, University of Southampton
Airfoil polars measured in the wind tunnel or simulated with XFOIL or other CFD software are usually limited to small and medium inflow angles (e.g. -20° to +20° AoA). However the blades of both VAWTs and HAWTs often operate at high inflow angles. In order for the BEM routines to converge and produce reasonable results it is essential that the airfoil performance polars (Lift, Drag…and Moment coefficients) are extrapolated to a 360° inflow angle range.
Traditionally the method of Viterna has been extensively used for this purpose. However B. Montgomerie of the Swedish Defense Research Agency has also developed a polar extrapolation method for wind turbine applications. QBlade includes both methods thus allowing the user to use the full extrapolation flexibility of both.
If you have the chance to play around with various extrapolation parameters within QBlade you will notice how critical the airfoil 360° polar extrapolation is for the validity and accuracy of the BEM simulation results.
1.1 Reference
MONTGOMERIE, B: Methods for Root Effects, Tip Effects and Extending the Angle of Attack Range to +-100deg, with Application to Aerodynamics for Blades on Wind Turbines and Propellers, FOI Swedish Defense Research Agency, Scien-tific Report FOI-R-1035-SE, 2004 VITERNA, L.A.; C ORRIGAN, R.D.: Fixed Pitch Rotor Performance of Large Horizontal Axis Wind Turbines, NASA Lewis Research Center, Cleveland, Ohio,1982
Footnotes:
Sheldahl, Robert E., and Paul C. Klimas. Aerodynamic characteristics of seven symmetrical airfoil sections through 180-degree angle of attack for use in aerodynamic analysis of vertical axis wind turbines. No. SAND-80-2114. Sandia National Labs., Albuquerque, NM (USA), 1981.
An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines—Part II: Post-stall Data Extrapolation Methods
49SA Ning. AirfoilPrep.py Documentation. Technical report, National Renewable Energy Laboratory, 2013.
50Larry A Viterna and David C Janetzke. Theoretical and experi- mental power from large horizontal-axiswind turbines. Technical Report NASA TM-82944, National Aeronautics and Space Ad- ministration, 1982.
Snel H, Houwink R, Bosschers J. Sectional prediction of lift coefficients on rotating wind turbine blades in stall. Report ECNC– 93-052, Energy research Centre of the Netherlands, 1994.
Viterna LA, Corrigan RD. Fixed pitch rotor performance of large horizontal axis wind turbines. DOE/NASA workshop on large horizontal axis wind turbines, Cleveland, Ohio, 1981.
Created: 2018-06-14 Thu 22:54
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